The Arsenal manager Arsène Wenger has been dealt a major blow after Abou Diaby was ruled out for nine months. The midfielder injured his left knee in training on Wednesday and scans have revealed that he has torn the anterior cruciate ligament, meaning he needs surgery and will be sidelined for the rest of the year.

Wenger had been resisting the temptation to rush Jack Wilshere back into action, despite a mounting shortage of midfielders at the club.

But the Diaby set-back leaves Arsenal with just Mikel Arteta, Aaron Ramsey and Francis Coquelin available for the central midfield berths.

Wilshere – who made his comeback in October after 17 months out injured – has not played since sustaining a badly bruised ankle in the 2-1 defeat to Tottenham Hotspur on 3 March and missed England's most recent World Cup qualifiers against San Marino and Montenegro.

Despite his absence, Arsenal have beaten Bayern Munich and Swansea City away in their only two fixtures since then. They have also moved to within four points of their north- London rivals Tottenham with a game in hand as they chase fourth place and Champions League qualification for a 16th successive season.

Yet with Diaby now sidelined after having played alongside Arteta against Swansea a fortnight ago, and Theo Walcott absent for tomorrow's meeting with Reading at the Emirates after picking up an injury on England duty, Wenger is taking a cautious approach with Wilshere due to concerns over his long-term future.

"We deferred his rehab a little bit by one week. We gave him one more week rest because it was wise medically to do it," Wenger said. "It was an inflammation of his bone and we had such history with him [that] we are a bit more cautious than we would be with you or with me.

"He will start his first run outside on Monday and six days after that will be too short. He's out for four weeks on Monday. He works very hard inside in the gym but it's not the same. He knows he has to take care of his future. Because he is a very young player, I think he accepts it."

Walcott is expected to be in contention for next week's visit to face West Bromwich Albion after pulling out of the England squad with a groin injury. He has failed to score in his last five appearances but Wenger was adamant that signing a new three and half year contract worth £100,000 a week in January has had nothing to do with his dip in form.

"I don't believe that. If he didn't play well and had not signed a contract, then people would say the opposite," he said. "I don't think that is linked especially with the fact that he has signed a contract or not. Because you can say as well he is more secure, he can express himself.

"I don't see any difference in attitude in training. He is a guy who loves football, who stays after training, works on his finishing. He was just a bit less decisive. He needs that fraction of a second to make the difference. He needs his pace."